ParkanPlayground/X86Disassembler/X86/Handlers/Bit/BsrR32Rm32Handler.cs

namespace X86Disassembler.X86.Handlers.Bit;

using Operands;

/// <summary>
/// Handler for BSR r32, r/m32 instruction (0F BD)
/// </summary>
public class BsrR32Rm32Handler : InstructionHandler
{
    /// <summary>
    /// Initializes a new instance of the BsrR32Rm32Handler class
    /// </summary>
    /// <param name="decoder">The instruction decoder that owns this handler</param>
    public BsrR32Rm32Handler(InstructionDecoder decoder)
        : base(decoder)
    {
    }

    /// <summary>
    /// Checks if this handler can decode the given opcode
    /// </summary>
    /// <param name="opcode">The opcode to check</param>
    /// <returns>True if this handler can decode the opcode</returns>
    public override bool CanHandle(byte opcode)
    {
        // BSR r32, r/m32 is a two-byte opcode: 0F BD
        if (opcode != 0x0F)
        {
            return false;
        }

        // Check if we have enough bytes to read the second opcode byte
        if (!Decoder.CanReadByte())
        {
            return false;
        }

        // Check if the second byte is BD
        var secondByte = Decoder.PeakByte();
        
        // Only handle when the operand size prefix is NOT present
        // This ensures 16-bit handlers get priority when the prefix is present
        return secondByte == 0xBD && !Decoder.HasOperandSizePrefix();
    }

    /// <summary>
    /// Decodes a BSR r32, r/m32 instruction
    /// </summary>
    /// <param name="opcode">The opcode of the instruction</param>
    /// <param name="instruction">The instruction object to populate</param>
    /// <returns>True if the instruction was successfully decoded</returns>
    public override bool Decode(byte opcode, Instruction instruction)
    {
        // Set the instruction type
        instruction.Type = InstructionType.Bsr;
        
        // Read the second opcode byte (BD)
        Decoder.ReadByte();

        // Check if we have enough bytes for the ModR/M byte
        if (!Decoder.CanReadByte())
        {
            return false;
        }

        // Read the ModR/M byte
        // For BSR r32, r/m32 (0F BD):
        // - The reg field specifies the destination register
        // - The r/m field with mod specifies the source operand (register or memory)
        var (_, reg, _, sourceOperand) = ModRMDecoder.ReadModRM();

        // Create the register operand for the reg field
        var destinationOperand = OperandFactory.CreateRegisterOperand(reg);

        // Set the structured operands
        instruction.StructuredOperands = 
        [
            destinationOperand,
            sourceOperand
        ];

        return true;
    }
}